Cyanobacteria, or blue-green algae, are commonly found as individual cells, clumps, filaments or large mats in Florida's lakes, rivers and estuaries.

Cyanobacteria blooms like this one can be very thick and resemble pea soup.

Cyanobacteria are some of the Earth’s oldest organisms, with fossils dating back 3.5 billion years. Yet, they can still be found today in all of Florida's freshwater and brackish habitats – lakes, rivers and estuaries. Like red tides, cyanobacteria can grow and accumulate, or bloom, when environmental conditions such as light availability and temperature are favorable. Nutrient pollution from agricultural and urban runoff causes the majority of freshwater cyanobacteria blooms. Other conditions that contribute to blooms are stagnant water resulting from a lack of natural flushing and land clearing. Cyanobacteria blooms can destroy submerged vegetation like seagrass by blocking sunlight. Blooms can also reduce oxygen availability to other aquatic organisms and introduce toxins that pass through the food chain. Toxins produced by cyanobacteria can be harmful to humans, affecting the liver (hepatotoxins), the nervous system (neurotoxins) and skin (dermatotoxins).

Several groups of toxic cyanobacteria have been detected in Florida's waters. The groups Microcystis, Anabaena and Cylindrospermopsis and their associated toxins – microcystins, anatoxin-a and cylindrospermopsin, respectively – all occur in Florida freshwater systems, including those used for drinking water. Persistent cyanobacteria blooms have affected many of Florida's aquatic systems, including Lake Okeechobee; the Harris Chain of Lakes (Apopka, Eustis, Griffin and Harris); and the St. Johns, St. Lucie and Caloosahatchee rivers and estuaries.

If ingested, water contaminated with toxic cyanobacteria can cause nausea, vomiting and, in severe cases, acute liver failure. In 2001, scientists discussed needs and methods for effective detection and treatment methods of toxins produced by cyanobacteria in drinking water reservoirs at the Cyanotoxin Detection and Quantification and Instrumentation Workshop. Although the presence of cyanobacterial toxins in reservoir systems used for drinking water is of potential concern in Florida, there have been no documented illnesses directly related to drinking water containing these toxins. Water treatment plants can effectively treat for one type of toxin – microcystins. These toxins can result in skin irritation, swollen lips, eye irritation, earaches, sore throats, hay fever-like symptoms (sneezing and runny nose) and fatigue after swimming in affected lakes.

The best way to prevent exposure to blue-green algae toxins is to avoid water where scum, foam or algae mats are present or where water is a greenish color. The Florida Department of Health offers these additional precautions:

Do not drink, cook or shower with untreated water from lakes, ponds or streams.

Do not allow pets or livestock to swim in or drink scummy water.

If you or your animals accidentally get into a blue-green algae bloom, wash with fresh water and soap after skin contact, and avoid swallowing or inhaling water. Wash animals' fur thoroughly before they start to groom themselves.

Avoid exposure to irrigation water drawn from untreated sources.

Notify your local water quality officials if you notice unusual changes in the taste or smell of your tap water.